This invention relates to a method and an apparatus for recording an image on a recording medium for a transmission image, and in particular, to an image recording method and an image recording apparatus in which an image composed of dots is formed on a recording medium which is movable in the sub-scanning direction by relatively moving a recording head with respect to said recording medium in the main scanning direction which is crossing said sub-scanning direction.
In recent years, in X-ray radiography, in place of an intensifying screen/film system (S/F), a system for picking up a digital electrical signal of an X-ray image such as computed radiography (CR) or a system employing a flat panel X-ray detector (FPD) has appeared. With the spreading of what is called a digital X-ray image pickup apparatus, also a digital medical image recording apparatus for recording a medical-use image on the basis of an electrical signal obtained by CR or an FPD system is spreading.
A recording method which has now become the greatest mainstream is a silver halide laser writing method in which an image is formed through converting an electrical signal of an X-ray image obtained by CR or an FPD system into laser beam intensity variation and carrying out print and development processing on a conventional silver halide film.
However, because the method uses a silver halide film in the same way as a conventional method, there is a problem that it is troublesome and costs much.
As regards a method not using a silver halide film, a thermal transfer method or a sublimation-type printer can be considered. However, in the case of a thermal transfer method, the ink of a recorded image is present on the uppermost surface of a film, which produces a trouble such that ink is easy to be transferred in handling. Further, in the case of a sublimation-type printer, sufficient density cannot be obtained and waste matter such as an ink ribbon is produced after image formation as in the case of a thermal transfer method.
Lately, an image recording apparatus employing an ink jet method has become versatile as a small-sized low-priced printer which enables the great improvement of the resolution and quality of a recorded image. Therefore, by applying an ink jet recording apparatus to X-ray image formation, the above-mentioned trouble is to be solved, and it is expected that an ink jet image forming method capable of forming an X-ray image which is made of low cost and easy to watch by making the most of the advantage of an ink jet printer can be provided.
In a medical-use image used mainly in diagnosis, in an image recording apparatus of not only an ink jet method but also all other recording methods, an extremely high image quality is required.
The reason is that because a medical-use image is always watched as a transmission image by putting it on a lighting box of a high illuminance in diagnosis, the density resolving power of human visual sensation becomes very much higher as compared to the case of a reflection image.
Further, a two-dimensional X-ray radiograph such that is radiographed by CR or an FPD system, what is called a simple X-ray radiograph, is basically a monochromatic image; in the case of a monochromatic image, because the density resolving power of human visual sensation is higher as compared to the case of other colors (for example, Y, M, C, etc.), a further higher image quality is required for a monochromatic transmission image.
Thus, in respect of indices to become the reference of image quality evaluation, which are the three items, namely, (1) gradation, (2) sharpness, and (3) granularity, an investigation concerning whether or not an image quality level required for a medical use can be achieved by an ink jet type recording apparatus has been practiced.
[Gradation]
It is said that the number of gray levels in a simple X-ray radiograph required for diagnosis is 10 bits (=1024 gray levels), and further, the number of gray levels enabling sufficient diagnosis is 12 bits (=4096 gray levels). In the case where an image of multiple gray levels such as a medical-use image is expressed by an ink jet method, because the number of ink density levels is limited, it is necessary to make the gradation expression of a recorded image in a digital way. For example, there is a method in which one pixel of image data is composed of a matrix having a plurality of elements, for example, a dither matrix of 4×4 elements, and gradation expression of 4×4+1=17 gray levels is made by using so called a dither method with this dither matrix made a unit.
Further, by using a plurality of kinds of ink, for example 4 kinds of ink, having colors of the same hue but different densities respectively, the number of gray levels to be produced can be increased innumerably.
However, actually it is general that gradation expression is made on the basis of an error diffusion method by selecting several to several tens of dither matrices out of all the dither matrices that are able to be produced and utilizing these several to several tens of dither matrices.
As regards the literatures concerning an error diffusion method, for example, it is described in detail in 'R. FLOYD & L. STEINBERG, “AN ADAPTIVE ALGORITHM FOR SPATIAL GRAY SCALE”, SID 75 DIJEST, pp. 36 to 37′. By using this gradation forming method composed of a dither method combined with an error diffusion method, multiple gray scale expression of 12 bits is possible, and by selecting suitable dither matrices and using a suitable error diffusion algorithm, it is possible to obtain a smooth gradation characteristic.
[Sharpness]
Sharpness, that is, the contrast of an image is important for a medical-use image having a purpose of diagnosis. For example, concerning an image of a foot, such a degree of sharpness as to make it possible to recognize trabeculae of bone clearly is desirable.
Because the unit of recording is an ink dot in an ink jet method, there is no factor to influence neighboring pixels other than the spreading of the ink dot diameter. Because there is no influence such as diffusion of light in a sliver halide laser writing method or remaining heat in a thermal transfer method, an image having a comparatively high sharpness can be obtained.
[Granularity]
Granularity, that is, the smoothness with no appearance of roughness is important for a medical-use image having a purpose of diagnosis. For example, concerning an image of a chest part, especially in a low density region, such a degree of granularity as to make it possible to recognize correctly the shade of a morbid portion etc. is desirable.
Because the unit of recording is an ink dot in an ink jet method, it sometimes occurs a case where the whole image is not covered with ink dots but a clearance is produced between dots. As the result, because sometimes the density appears low in general, or the image appears rough, the granularity appears rather bad.
One method of reducing granularity is to jet fine ink particles in extremely high density. However, in case of fine particles, in order to cover the whole area of the image without clearance it is necessary to jet ink particles in almost the same area. To enable this, the ink absorbing speed and the ink absorbing amount of the recording media need to be improved. Further, for a medical-use image being used for a diagnoses which is desirable to have the maximum output image density of 3.0, in order to achieve the maximum image density of 3.0 by ink jet printing, a large amount of ink is required to be used.
On the other hand, as regards a means for improving granularity, a method in which the dot diameter is made larger to eliminate the clearances between dots can be considered; however, on the contrary, in some cases it occurs a phenomenon, what is called “beading”, which is a phenomenon such that neighboring ink dots are coupled, and the granularity becomes rather bad. Further, because the tendency to produce the beading varies in accordance with the external environment such as the room temperature, a stable output density is not always obtained, and as the result of it, sometimes gradation characteristic is lowered. Further, if the dot diameter is made excessively larger, sometimes recorded image is blurred and degradation of sharpness is brought about.
This invention has been made in view of the above-mentioned problems, and it is its object to provide an image recording method and apparatus capable of obtaining a high-quality print image with the suppressed image deterioration caused by the fluctuating factors in recording such as the dot diameter of ink and the deviation of landing position of an ink drop.